A lot of left-criticism of Bernie Sanders is beginning with an odd but telling assumption: that he is running for president. This is only colloquially true. At this stage in the process, when we say that Sanders is running for president, that's just rhetorical shorthand for what is actually happening: he's running for the Democratic nomination. Then and only then will he possibly run for president.

The distinction is important, because at this stage he is only running against Hillary Clinton and whomever else decides to compete for the nomination. He is not running against Green Party candidates like Jill Stein or Cynthia McKinney. He is not running against whomever the SPUSA eventually nominates. He is not running against the radical with much better politics who is going to get swept into office on a wave of popular acclamation in a year or so.

Maybe you think that a Sanders presidency would co-opt popular support for socialism towards another four years of neoliberal hegemony. This is a good reason to oppose a Bernie Sanders presidency, but it doesn't necessarily follow that you should therefore also oppose a Sanders nomination. For instance, one could argue that a truly radical opponent would be more likely to beat Sanders than Clinton in the general election - he is, after all, a far weaker candidate. Perhaps a Sanders nomination would be a better way of bringing into sharp relief the contrast between Socialism In Name Only and a real, principled socialist agenda.

How you come down on such matters is mostly a question of strategic calculus. It does not necessarily have anything to do with supporting a Sanders presidency.